Modulation of cyclonic disturbances over the north Indian Ocean by Madden - Julian oscillation

Abstract

The relationship of genesis and intensity of cyclonic disturbances (CDs) over the north Indian Ocean with the Madden – Julian Oscillation (MJO) has been examined using 33 years (1975 - 2007) data of MJO index and best track of (CDs) developed by India Meteorological Department (IMD). The MJO index based on outgoing long wave radiation (OLR) and zonal wind in upper (200 hPa) and lower (850 hPa) troposphere (Wheeler and Hendon, 2004) has been used for this purpose.

The MJO strongly modulates the genesis and intensity of CDs over the north Indian Ocean. However there are other factors contributing to cyclogenesis over the north Indian Ocean, as about 60% of cyclogenesis during monsoon and post-monsoon seasons are not significantly related with MJO. While the probability of cyclogenesis during monsoon season is higher with MJO in phase 4 and 5 (Maritime Continent), that during post-monsoon season is higher with MJO in phase 3 and 4 (east Indian Ocean and adjoining Maritime Continent). It indicates that while possibility of genesis during monsoon season is significantly suppressed with active MJO at phase 1, 7 and 8 (Africa, western Hemisphere and adjoining Pacific Ocean), there is no significant relationship between genesis and active MJO at phase 1, 7 and 8 during post-monsoon season. The anomalous cyclonic circulation at lower levels over central and north Bay of Bengal in association with MJO at phase 4 and 5 favours enhanced probability of cyclogenesis over the Bay of Bengal during monsoon season. The anomalous easterlies in association with MJO at phase 1 and development of anomalous ridge over south India in association with MJO at phase 7 and 8 which are weak monsoon features lead to suppressed cyclogenesis over north Indian Ocean during this season. The anomalous north-south trough in easterlies embedded with cyclonic circulation over the south west/west central Bay of Bengal in association with southerly surge over the region during active MJO in phase 3 and 4 most favourably influences the convection and enhances the probability of cyclogenesis over the north Indian Ocean during post-monsoon season.

The genesis of CDs is more sensitive to phase than the amplitude while the intensification of CDs is more dependent on the amplitude of MJO. Comparing monsoon and post-monsoon seasons, the modulation of genesis, intensification and duration of CDs by the MJO is more during the monsoon season than the post-monsoon season.